I don't know of the exact changes between the 07/08 but as for the X51 I think it is worth it overall. I would rather have a performance upgade by Porsche done to my car at the factory then have one done elsewhere. Even if you go to an aftermarket place and get more hp or a lower price I would perfer to have the one by Porsche. That is just my feeling. Now is the X51 worth 17K US? My answer is no but I don't think the 997 is worth 100K either. But even with its high price I still bought one
If I was in the market and price was no object I would love to have a C4S Cab with X51. That would be killer. Now your price is going to get close to a 997TT but to me the drop top is the way to go. I sure many will disagree